Thermoplastic Sandwich Materials
The stiffness of thermoplastic sandwich materials is due to the distance between its two skins that gives a high moment of inertia to the part. It is therefore important to preserve the distance between the skins during the molding of the sandwich part in order to obtain the full stiffness potential of the part. Any reduction of thickness causes a reduction of stiffness.
During the conception of a sandwich part with a cellular core, it is sometimes necessary to reduce the thickness of the part locally in order to obtain a specific shape. The stiffness in that area will be reduced but can be maintained by the part geometry. An example is that of a suitcase corner that gives the stiffness to the suitcase even though the part thickness is uniform.
Thermoplastic sandwich materials are traditionally used as flat panels or shaped parts for the following applications:
Flat PanelsShaped PartsIndustrial VehiclesAir and Space,Building ConstructionBoats TrainsMaterialsAllThermosetsFormability−−++Volumes+−Cost++−−Aspect++Currently, the market for sandwich materials is:                Competitive cost for flat panels,        Very high cost and low volume for shaped sandwich parts limits market to high technology applications and not high volume parts such as automotive parts.Deep-Draw Stamping of Steel Sheets        
A stamped steel part is deep drawn wherein the depth of the part is typically more than one hundred times its thickness. Deep-draw of steel is possible because of its ductility.
Technical solutions were developed such as the use in the mold of a blank holder at the periphery of the stamping area that allows the part to be deformed under a high pressure without any folds. Folding the excess material of the sheet rather than stretching it is avoided because the created folds are brittle in fatigue and aging.
The pressure of the blank holder is a function of:                Initial blank diameter and its thickness after stamping;        Tensile strength of the steel;        Diameter of the stamping mold; and        An empirical parameter, n, that is a function of the ratio of blank diameter to stamping die diameter.Stamping of a Cylindrical Steel Part        
To manufacture cylindrical parts starting from a steel blank, it is possible to obtain a part with a maximum depth that is equal to seven times its diameter but six successive stampings are needed.
As an example, to obtain a cylindrical part with a diameter of 100 mm and a depth of 700 mm, eight stages are required as follows:                1. Cut a circular blank of 538 mm diameter;        2. Stamp to obtain a part with a 325 mm diameter and a depth of 145 mm;        3. Stamp to obtain a part with a 240 mm diameter and a depth of 240 mm;        4. Stamp to obtain a part with a 194 mm diameter and a depth of 325 mm;        5. Stamp to obtain a part with a 155 mm diameter and a depth of 428 mm;        6. Heat treat the part;        7. Stamp to obtain a part with a 124 mm diameter and a depth of 553 mm; and        8. Stamp to obtain a part with a 100 mm diameter and a depth of 700 mm.        
After the fifth stage it is necessary to heat treat the part to regenerate the metal.
Stamping of a Part With Four Corners
The technique is similar to that used for cylinders except for the shape of the initial blank to avoid too much material loss after stamping. To define the development of the corners and of the walls, curves based on experience exist to define the depth and the radius.
Stamping of Sandwich Steel Blanks
Sandwich steel blanks are made of a sandwich with thin skin layers with a core. The blanks can be stamped.
Processing of Flat Thermoplastic Sandwich Materials
European Patent EP 0 649 736 B1 explains the principle of the sandwich technique for forming substantially flat parts. The part is made in a single stage by pressing in a cold mold, at a pressure in the range of 10 bars to 30 bars, a stack consisting of at least a first outer skin layer of stampable reinforced thermoplastics material, a cellular core of thermoplastics material, a second outer skin layer of stampable reinforced thermoplastics material. The axes of the cells of the cellular core are generally oriented perpendicular to the skin layer. The skin layers and core are previously heated outside the mold to a softening temperature.
European patent application 894611 A1 describes a 2D deep-draw which utilizes an element such as a piece of EPP foam to make sure that a skin is not in contact with a core during heating. When the blank is stamped, the tension on the skin is such that it does not crush the core during forming.
Shaping of Sandwich Material By 3D Deep-Draw
Solutions Used for Sandwich Material in the Aeronautics Industry
These solutions consist in erasing the problems due to:                The high stiffness of the skins in elongation,        Low compression strength of the core as compared to the tensile strength.        
Therefore, to form a corner in a sandwich part, Hexcel Composites Company cuts the skin, and adds epoxy adhesive to fill the honeycomb.
If the corner will be exposed to high stress, the solution in the aeronautics industry is:                either the corner is reinforced with an angled insert(s) that is glued, or        the corner is reinforced with a metallic insert (extruded) with which two sandwich panels are assembled (glued).Deep-Drawing of Fabric-Reinforced Thermoplastics        
Some experimental work has been done at the German Institute IVW (Kaiserslautern Germany) on the deep-drawing of fabric reinforced thermoplastics. A summary of that work is presented in the journal POLYMER COMPOSITES, August 1996, Vol. 17, No. 4, p. 643–647 by Breuer, Neitzel, Ketzer and Reinicke. They explain that for stamp forming of fabric-reinforced sheet material into simple 3D parts ordinary planar blank holders have proved to be sufficient to avoid any wrinkling. However, to stamp more complex shaped and large parts, this technique is limited. In this case, different clamping forces are needed on different areas of the blank. They propose a flexible roller-tracking device where the sheet material is drawn into the mold between two knurled rollers. With this system, different tension forces can be applied at selected zones of the sheet.